Elm tree called Regal

ABSTRACT

An elm tree characterized by high resistance to the Dutch elm disease.

The present invention relates to a new and distinct variety of elm treewhich is primarily distinguished by its high resistance to Dutch elmdisease caused by Ceratocystis ulmi. The tree has remained symptomlessin environments known to contain Verticillium dahliae, the causal agentof Verticillium wilt and appears to have limited susceptibility to blackleaf spot disease caused by Stegophora ulmea.

The tree is characterized by a columnar growth habit. Soon afterplanting it develops a strong central leader and the young plantingsrequire a minimum of care to maintain a strong, upright (columnar)growth habit even when subjected to partial shading. Development ofolder plantings (10 years) in good sites suggests a long-lined, tidytree which will perhaps reach heights of 20 m or greater.

Foliation of the tree tends to be less dense than in the American elmwith shade density resembling more closely trees such as thornless honeylocust (Gleditsia triacanthas L.) or little leaf linden (Tilia cordataMill.)

The foliation and uniform upright growth characteristics of the treeallow full lawn development while maintaining an attractive columnarhabit. Thus the tree is an ideal candidate for planting in lines alongdrives or roads or in modern urban settings such as shopping malls andhousing developments. Additionally, ice storms or severe winds have notproduced injury in the parent tree under conditions causing large branchdamage to U. pumila.

New spring growth is varleys-green in color (R#31'm) (Ridgway, 1912). Asleaves mature they change to a glossy, trichome-free dusky yellowishgreen (R#27'"m) and then to a mature forest green (R#29'm). The lowerleaf surface is a deep, dull yellow greeen (R333"k). Trichomes on thelower leaves are not prominent and are confined to the bases of theveins. Mature leaves exposed to full sun are lanceolate-ovate, unequalat the base, acuminate at the tip, doubly serrate at the leaf margins,8-12 cm long and 3 to b 6 cm wide. The short petioles vary from 0.8 to1.5 cm in length. Fall leaf color is not distinctive and leaves tend tobe retained through several early frosts before abscission occurs. Thischaracteristic is common in certain sources of U. pumila growing insouth-central Wisconsin.

Apical dominance is very strong and results in limited side branching onthe developing shoot of the current season's growth. Current season sidebranching is usually restricted to short spurs which continuesubdominant growth the following season. This results in the stiff,upright growth and resulting light density foliage.

Young branches are a light brownish gray (light brown drab R#9""b) withgreen showing through the bark cracks. Prominent brownish lenticels 1.3to b 1.5 cm long form an attractive pattern on the larger branches andlower trunk of young trees. This pattern is broken at regular (2 to 2.5cm) intervals by horizontal brownish-green bark cracks. Mature bark is adark neutral to mouse gray (R315""') and the substance bark is colored alight Sanford's brown (R#11k). Mature bark development occurs relativelylate in the tree's life and only the basal portions of specimens atleast 8-10 years old have appreciable mature bark development.

The parent tree is located in a dense planting of elm sections, andthese conditions have generally retarded its growth as contrasted withother plantings on more fertile urban planting sites. The parent treebegan flowering sporadically at about 15 years of age, but trees fromrooted cuttings have flowered in 10 years. Floral and seedcharacteristics are not distinctive and most closely resemble its strongU. carpinifolia ancestry.

The accompanying photographs show a specimen tree of the new variety inwinter and in full foliage. The specimen shown is approximately nineyears old and has an approximate height of 5 meters and an approximatediameter of 20 centimeters. The columnar growth habit is readily seenfrom the photographs. There is no tendency for the branches nearer thebase of the tree to spread and branches emanating from the trunk tend toremain relatively uniform in size.

ORIGIN

The "Regal" elm is derived from a controlled pollination of the"Commelin" elm N274 (U.×Hollandica vegeta×U. carpinifolia #1) with N215(U. pumila×U. carpinifolia hoersholmiensis).

The hardiness (survival in Wisconsin winters) and disease resistance maybe attributed in part to its 25% U. pumila ancestry as well as thenorthern (Denmark) origin of its U. carpinifolia hoersholmiensisgrandfather.

ASEXUAL REPRODUCTION

The "Regal" elm cultivar can be propagated from root cuttings allowed tosprout in the greenhouse in moist sphagnum peat and sand.Hormone-treated sprouts can be transplanted as rooted cuttings afterapproximately 20 days in a perlite-peat rooting medium under fineintermittent mist. This is the preferred method of propagation.Bud-grafting has been of limited success on a range of root stocks.

DISEASE RESISTANCE

Methods for screening elm introductions and breeding progeny for DEDresistance have been previously described (Smalley and Kais, 1966, in H.D. Gerold et al (eds.) Breeding Pest Resistant Trees, Pergamon Press,New York, pp. 279-292; Lester and Smalley, I.U.F.R.O. Genetics-SABARAOJoint Symposia, Tokyo, 1972, C-5(V):1-10). In general, seedlings aregrown for one year in out-door seedbeds, transplanted into fieldlocations the second season, maintained under clean cultivation, andartificially inoculated with Certocystis ulmi in the 3rd and 4th year.Initial screening trials of plants from the Dutch seed lots from which"Regal" was derived utilized 10-15 different Wisconsin isolates of thepathogens. Trees were inoculated in one-year-old wood in the tree'supper crown and the inoculum contained approximately 10⁶ spores/ml.Survivors of this procedure were re-inoculated in subsequent years. As afinal test (called the seasonal susceptibility test), selected resistantclones were clonally propagated in large numbers, grown in randomizedfield plants for one or two years, and different individuals of eachclone were inoculated at 10 different times during the growing season(Table 1). In this test, the inoculum consisted of a mixed conidialsuspension from 10 different isolates of the fungus from different NorthAmerican locations. Observations on disease development were thenrecorded periodically through the year following inoculation. Thecultivar herein described, named "Regal", possessed superior resistanceto C. ulmi as shown in Tables 1 and 2.

                  TABLE 1                                                         ______________________________________                                        Seasonal susceptibility of selected elm clones to Dutch elm disease.          Percent crown damage at final reading (7/17/74).sup.ab                                     Clone                                                            Elm species.sup.c                                                                          Number   1973 dates of inoculation                               ______________________________________                                                              5/17   5/24 5/31 6/7  6/17                              ______________________________________                                        Ulmus americana                                                                            185-2    22.2   0    29.4 0    0                                              185-4    33.3   62.5 33.3 41.3 11.0                              Ulmus pumila 44-26    0      0    0    0    0                                 ×      44-27    0      2.5  0    0.1  0                                 Ulmus japonica                                                                Ulmus hollandica                                                                           5-2      82.0   83.8 90.5 25.5 38.8                              ×                                                                       Ulmus pumila                                                                  Ulmus carpinifolia                                                                         134-2    6.0    11.1 12.5 0.1  8.3                                            380-1    0.2    0    0    0    0                                 Ulmus pumila 196-5    0      0    0    0    0                                 ×      196-6    1.4    0    9.5  3    23.3                              Ulmus carpinifolia                                                                         196-7    0      13   10.0 15.5 16                                             196-8    0.4    0    0    1.3  0                                 Ulmus (complex.sup.c                                                                       228-5    0/3    0/6  0/3  0/3  0/4                               hybrid) `Regal`                                                               ______________________________________                                                              6/21   6/28 7/5  7/19 8/2                               ______________________________________                                        Ulmus americana                                                                            185-2    0      0    0    0    0                                              185-4    0      26.7 0    0    0                                 Ulmus pumila 44-26    0      0    0    0    0                                 ×      44-27    0      0    0    0.1  0                                 Ulmus japonica                                                                Ulmus hollandica                                                                           5-2      36.5   13.2 26.7 14.4 7.5                               ×                                                                       Ulmus pumila                                                                  Ulmus carpinifolia                                                                         134-2    12.5   0    18.7 0.4  16.7                                           380-1    1.0    0    0.2  0    0.2                               Ulmus pumila 196-5    0      0.1  0.6  0.8  0                                 ×      196-6    31.8   7.4  10.0 0.5  1.3                               Ulmus carpinifolia                                                                         196-7    11.3   0    1.7  0.3  0                                              196-8    0      1.3  1.5  1.3  0.6                               Ulmus (complex.sup.c                                                                       228-5    0/6    0/1  0/6  0/4  0/5                               hybrid) `Regal`                                                               ______________________________________                                         .sup.a Elm clones derived from trees previously symptomless following one     or more inoculations with Wisconsin strains of Ceratocystis                   .sup.b Mixed inoculum from isolates of C. ulmi from Wisconsin, Kansas,        Massachusetts, Illinois, Nebraska, Minnesota, New York, Conneticut, and       Maine.                                                                        .sup.c Trees planted as rooted cuttings in 1971, 3 plants per clone per       block, 15 clones per block, and 40 blocks (some clones incompletely           represented in blocks because of transplant mortality or shortage of          plants). Not all clones in the study are shown.                          

                  TABLE 2                                                         ______________________________________                                        Response of elm clones to inoculation in the field with an                    aggressive and a nonaggressive isolate of Ceratocystis ulmi                   and their single ascospore progeny.                                           Clone     Crown damage (%).sup.a                                              Elm   num-    Ascospore lines       Parental                                  species                                                                             ber     1     2   3   4   5   6   7   10  11  99.sup.b                                              116.sup.c                                         ______________________________________                                        Ulmus 412-1   48    44  83  91  33  81  81  --  --  80                                                    97                                                                            ameri- 411-3 -- 27 78 78 -- 84 78 32 -- 95 99                                 cana                                                                          Ulmus 44-11.sup.d 0 0 0 0 0 0 0 0 0 0 0                                       pumila 44-26 0 0 0 0 0 0 2 -- 2 2 0                                           × 44-27 0 0 0 0 0 0 0 0 0 0 0                                           Ulmus                                                                         japon-                                                                        ica                                                                           Ulmus 5-2 14 0 1 1 0 0 33 1 -- 67 2                                           hol- 5-2 2 0 5 0 1 -- -- -- -- 7 6                                            landica                                                                       ×                                                                       Ulmus                                                                         pumila                                                                        Ulmus 368-2 37 0 19 25 0 15 -- -- -- 62 32                                    carpini- 368-8 19 0 14 -- -- -- -- -- -- 97 15                                folia 368-10 25 2 24 -- -- -- -- -- -- 95 17                                  Ulmus 196-5 0 0 -- 1 12 0 3 -- -- 7 0                                         pumila 196-6 0 0 0 0 0 0 0 0 2 5 0                                            × 196-8 0 0 0 0 0 0 2 0 -- 0 0                                          Ulmus                                                                         carpini-                                                                      folia                                                                         Ulmus 228-5 0 0 0 0 0 0 0 -- -- 0 0                                           (com-                                                                         plex.sup.c                                                                    hybrid)                                                                       `Regal`                                           ______________________________________                                         .sup.a Each value is a mean of six replications (trees) inoculated in         early June, 1975. Data adapted from Gkinis, 1977.                             .sup.b Aggressive isolatetype B from Minnesota                                .sup.c Nonaggressive isolatetype A from Maine.                                .sup.d Sapporo Autumn Gold elm.                                          

What is claimed is:
 1. A new and distinct variety of elm tree calledRegal as herein described and illustrated primarily characterized by itshigh resistance to Dutch elm disease and its columnar growth habit.